This is a really neat method to include CSS styles into an HTML email inline, which is apparently the best practice for HTML emails.
This is awesome because you can keep the seperation between presentation and content, and simply integrate a stylesheet into an HTML email automatically just before sending. As a basic example without changing Mark's code at all:
$email = new ProcessedEmail(
$from = '',
$to = '',
$subject = '',
$body = ''
$css = file_get_contents(Director::getAbsFile($this->ThemeDir().'/css/Email.css'));
'InlineCSS' => "<style>$css</style>"
Then just include $InlineCSS in the email template and the styles will be made inline automatically when ProcessedEmail-> parseVariables() is called. Very cool!